1. Field of the Invention
The invention relates to glasses for use in lighting means, especially in the area of background illuminations.
2. Description of the Related Art
It is known that quite special requirements are placed on the glass and its qualities for background illuminations. This concerns the small dimensions of the lighting means used, the correspondingly extremely low thicknesses as well as the UV-absorbing qualities, in which case the irradiated high-frequency energy should not be absorbed or should be absorbed only to a slight extent by the lamp glass in order, for example, to cause the illuminating gas enclosed in the fluorescent lamp to ignite.
Furthermore, the use of alkali-free glasses as a component of background illuminations is known. For example, utility model DE 20 2005 004 459 teaches alkali-free glasses that are specially adapted for use in lighting means with external electrodes. They are for example, EEFLs (external electrode fluorescent lamps), also designated as electrodeless discharge lamps since there are no electrode lead-throughs but rather only external or outside electrodes.
DE 103 06 427 A1 describes the use of aluminosilicate glasses, especially alkaline-earth aluminosilicate glasses, with a transformation temperature Tg>600° C. for the manufacture of lamp bulbs for discharge lamps. The glass composition includes >55-64 wt. % SiO2, 13-18 wt. % Al2O3, 0-5.5 wt. % B2O3, 0-7 wt. % MgO, 5-14 wt. % CaO, 0-8 wt. % SrO, 6-17 wt. % BaO, 0-2 wt. % ZrO2 and 0-5 wt. % TiO2. This glass was developed especially for discharge lamps with external contacting and has a high thermal resistance.
Furthermore, DE 10 2005 000 664 A1 describes the process for adjusting the UV absorption of glasses and glass ceramic materials in which the lowest possible TiO2 content is present in the glass. The glasses are designed in particular for EEFL applications, that is, lighting means with external contacting.
DE 10 2006 005 611 A1 relates to a display with background illumination including lighting means with external electrodes in which the total efficiency of the lighting means is optimized by a purposeful selection of the glass composition.
However, the above-described glasses have very high glass transformation temperatures Tg, for example, Tg>600° C., preferably Tg>700° C. and frequently a high Al2O3 content in order to achieve a high temperature stability. This high temperature stability is actually not necessary for the use as casing tube of a low-pressure glass discharge lamp and is not favorable in every case in the manufacture of the lamps. Thus, high processing temperatures are required for melting in the electrode lead-throughs and for closing the tubes by melting. This results in a high energy consumption associated with high expenses. Furthermore, the described glasses are suitable for lamps of the EEFL type and are adapted to the thermal expansion behavior of metal/metal alloy of the electrode lead-throughs (longitudinal expansion, CTE), but not to other lamp types, such as, e.g., CCFL.
It is furthermore known that alkali-containing borosilicate glasses specially adapted for only one lamp type are customarily used for CCFL (cold cathode fluorescent lamp) or also EEFL (external electrode fluorescent lamp) gas discharge lamps, which glasses have the advantage that they can be worked at relatively low temperatures and can be adapted very well to the coefficients of expansion of the metals/ alloys of the electrode lead-throughs (e.g., molybdenum, Kovar).
For example, an alkali-containing silicate glass for a fluorescent lamp is known from the state of the art in accordance with JP 01-239037, which glass has the following composition range: 65-75 wt. % SiO2, 0.5-2.5 wt. % Al2O3, 1.0-5.0 wt. % MgO, 3.0-8.0 wt. % CaO, 5.5-9.5 wt. % MgO+CaO, 13-19 wt. % Na2O, 0-3.0 wt. % K2O, 13.0-20.0 wt. % Na2O+K2O, 0.3-3 wt. % B2O3, 0.1-1 wt. % P2O5, 0.4-0.8 wt. % Sb2O3 and 0.03-0.05 wt. % Fe2O3. The glasses have improved transmission and workability as well as a reduction of the manufacturing costs and should be more environmentally friendly.
However, the described alkali-containing glasses have the disadvantageous tendency to the so-called blackening, a darkening of the glass by the reaction of mercury with the alkali components of the glass, especially sodium. Furthermore, these glasses have poor dielectric qualities for use in EEFL applications, which is reflected, for example, in a too high quotient of tan delta/relative permittivity, and therefore signifies a poor efficiency. Furthermore, the puncture strength of the glasses, so-called pinhole stability or pinhole burning, which signifies a puncturing at high voltages, is low.
What is needed in the art is to avoid the disadvantages of the state of the art and to make a glass available that is suitable for meeting the desired requirements in the area of background illuminations to a high degree and which is equally suitable for applications in lighting means with external as well as internal contacting.